A closed-loop lambda control is, in connection with a catalytic converter, today the most effective emission control procedure for the gasoline engine. Low exhaust gas values can be achieved only through interaction with ignition and injection systems available today. The deployment of a three way catalytic converter or a selective catalytic converter is especially effective. This type of catalytic converter has the quality to reduce hydrocarbons, carbon monoxide and nitrogen oxides up to more than 98% in the instance that the engine is driven in a range of approximately 1% around the stoichiometric air-fuel-ratio with LAMBDA=1. In so doing, the variable LAMBDA also denoted as “air number” indicates how far the actual air-fuel-mixture at hand deviates from the value LAMBDA=1, which corresponds to a theoretically necessary mass ratio for complete combustion of 14.7 kg air to 1 kg gasoline, i.e. LAMBDA is the quotient from the air mass delivered and the theoretical air supply, which is needed.
In the closed-loop lambda control the respective exhaust gas is measured and the amount of fuel delivered is corrected immediately corresponding to the measurement result by means of, for example, the fuel injection system. A lambda sensor is used as a probe, which has a voltage jump exactly at LAMBDA=1 and in this way delivers a signal, which indicates if the mixture is richer or leaner than LAMBDA=1. The mode of operation of the lambda sensor is based on the principle of a galvanic oxygen concentration cell with a solid electrolyte.
Lambda sensors designed as two-point sensors work in an inherently known manner according to the Nernst principle, and in fact based on a Nernst cell. The solid electrolyte consists of two border surfaces separated by a ceramic surface. The ceramic material used becomes conductive for oxygen ions at approximately 350° C., so that the so-called Nernst voltage is produced then on both sides of the ceramic surface when the oxygen proportion is different. This electrical voltage is a measurement for the difference of the oxygen proportions on both sides of the ceramic surface. Because the residual oxygen content in the exhaust gas of an internal combustion engine is dependent to a great degree on the air-fuel-ratio of the mixture delivered to the engine, it is possible to use the oxygen proportion in the exhaust gas as a measurement for the actual existing air-fuel-ratio.
In the so-called wideband sensors, the probe is designed as the wideband sensor. This is formed from solid electrolyte layers as well as from a number of electrodes. Such a construction proceeds from the German patent DE 19 912 102 A1, especially from the pages 8 and 9, which lie therein next to FIG. 1. The context of the patent at hand makes full reference to the aforementioned DE 19 912 102 A1. These electrodes are schematically reproduced in the subsequently described FIG. 1. A part of the designated electrodes form a so-called pumping cell in this sensor. The other part forms a so-called concentration cell. Furthermore, a first cavity is configured by the solid electrolyte layers (subsequently “gas measurement chamber”).
A pumping voltage is applied to the electrodes of the pumping cell, by means of which in a first gas measurement chamber a constant oxygen partial pressure, i.e. a corresponding air number LAMBDA, is adjusted by additionally pumping oxygen in or out. In so doing, the pumping voltage is controlled in a closed-loop in such a way that a constant voltage value of 450 mV appears at the electrodes of the concentration cell. This voltage corresponds to a value of LAMBDA=1.
In wideband lambda sensors according to the designated double cell principle, the air number in the gas measurement chamber of the pumping cell is closed-loop controlled to a certain value, which preferably is maintained constantly at LAMBDA=1. The air number in the gas measurement chamber of the pumping cell is specified by the designated comparison voltage, which is generated by the control unit of the internal combustion engine for the Nernst cell.
A diffusion barrier lies in front of the Nernst cell. Each gas diffusing through the diffusion barrier causes a pumping current via the designated closed-loop control due to the change of the gas composition in the designated gas measurement chamber and the change of the Nernst voltage connected with it. This pumping current represents a measurement for the partial pressure difference, the diffusion coefficient and the oxygen requirement per molecule of the gas in question.
In internal combustion engines with self-ignition using afterinjection of fuel for the purpose of regenerating a particle filter disposed in the engine, the wideband sensor must be able in the lean operation to simultaneously detect the oxygen and the rich fuel. On the basis of the different diffusion coefficients of both of these gases, the relatively heavy rich gas (HC) is less significantly evaluated for its oxygen requirement as the oxygen, and as a result hydrogen is too significantly evaluated. The same is true for a mixture of rich gases, which, for example, occur in an externally-supplied ignition of an internal combustion engine in the rich operation or during regeneration of a storage catalytic converter in an internal combustion engine with self-ignition. For this reason, an evaluation of the adjusted LAMBDA-value is only possible with knowledge of the proportion of HC, respectively H2. With the known lambda sensors, only the partial pressure of one of the gas components can consequently be correctly measured.
Because the known lambda sensors deliver only an output signal (and in fact the designated pumping current), the information cannot simultaneously deliver the partial pressure and the gas composition. For this reason, it is desirable to provide a procedure for the operation of one of the lambda sensors here in question. By means of this procedure, the partial pressure and the gas composition can be simultaneously determined.